As an electrolyte for batteries, liquid or gel electrolytes have heretofore been used because of their ionic conductivity. An outer casing having a high mechanical strength must be used for batteries to avoid liquid leakage leading to damage of instruments. This poses a problem, for example, interferes with miniaturization or weight reduction of batteries.
To solve the above-mentioned problem, solid polymer electrolytes are being examined. Solid polymer electrolytes have good processability and pliability and therefore batteries can have various shapes without restriction. Further, solid polymer electrolytes have no fluidity and hence batteries possess a high safety.
For example, an attempt has been made proposed wherein an ethylene oxide-propylene oxide copolymer having incorporated therein a specific alkali metal salt is adopted as ion-conductive solid electrolyte (Japanese Unexamined Patent Publication [hereinafter abbreviated to “JP-A”] No. S61-83249, JP-A S63-136407 and JP-A H2-24975). However, higher ionic conductivity and higher mechanical strength are now desired. Further, solid polymer electrolytes used for batteries are handled usually in the form of film in the course of production. Therefore, it is required that solid polymer electrolytes have good film-forming property, and the film thickness is rendered as thin as possible to enhance the capacity of battery.
A solid polymer electrolyte must have high mechanical strength for the reduction of film thickness. For this purpose, a solid polymer electrolyte used is prepared by forming a film from a polyether polymer having crosslinkable reactive functional groups, and curing the film to effect crosslinking. For example, it is proposed in JP-A 2000-123632 that a highly polymeric solid electrolyte is prepared by copolymerizing ethylene oxide and propylene oxide with an oxirane compound having a functional group such as ethylenically unsaturated group, a reactive silicon-containing group, an epoxy group or a halogen group to produce a ring-opened polyether copolymer, and then, curing the polyether copolymer to effect crossinking. It is described in this patent publication that the crosslinked product obtained from the ring-opened polyether copolymer has good processability, good formability and high mechanical strength.
However, processability and formability are incompatible with mechanical strength, and hence, a crosslinked product having good and well-balanced processability, formability and mechanical strength is difficult to obtain.
Further, when the ring-opened polyether copolymer is pd, another problem arises in that, since a monomer mixture containing a crosslinkable monomer is polymerized, crosslinking tends to undesirably occur during polymerization to give a copolymer containing a large amount of crosslinked product. The copolymer containing a large amount of crosslinked product has poor evenness of mechanical strength, and a thin film is difficult to produce from the copolymer by casting or extruding procedure. Further, crosslink density is not uniform in the film, and hence ionic conductivity is not uniform over the entire film. The film has poor surface smoothness, and hence, dendrite tends to be produced on the surface when charge and discharge of a battery are repeated.
A secondary battery provided with a solid electrolyte composed of the above-mentioned film made of the crosslinked product-containing polyether copolymer exhibits a considerably large reduction of battery performance with the lapse of time when charge and discharge of the battery are repeated.